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1.3.3 Cosmological Redshift
The third alternative, which is often referred to as the cosmological hypothesis, has occupied the central place in this field. In this hypothesis our Galaxy, along with other galaxies and any other extragalactic objects, is located in an expanding universe and the redshift of any such object is explained as arising from the time-dilation produced in the curved space-time of such a universe. This hypothesis explains the Hubble law (see Chapter 2) observed for galaxies, namely that the redshift of a source is proportional to its distance from us. Right from the early days this has been the favourite hypothesis for quasars also, for the following reasons.
The redshifts of quasars, although by and large greater than those of galaxies, seem to arise as a natural consequence of the expanding universe hypothesis.
If this is true, no detailed structural scenarios are needed (as in the other two options discussed above) to understand the redshift. Quasar properties are in many respects similar to those of the nuclei of Seyfert galaxies, and of active galactic nuclei in general. Since the redshifts of these galaxies and nuclei are believed to be cosmological, the redshifts of quasars can likewise assumed to be so.
All the same there still persist some doubts about the universality of the cosmological hypothesis and we will return to the contentious issues in Chapter 15. For the time being, and in the following chapters, we will assume that this hypothesis holds for the quasars.